Pressure vessel steel plate having excellent post weld heat treatment resistance, and manufacturing method therefor

1. A pressure vessel steel plate having excellent resistant against a post weld heat treatment (PWHT), the pressure vessel steel plate comprising 0.05 to 0.17 wt % of C, 0.59 to 1.00 wt % of Si, 0.3 to 0.8 wt % of Mn, 1.0 to 1.5 wt % of Cr, 0.3 to 1.0 wt of Mo, 0.003 to 0.30 wt % of Ni, 0.003 to 0.30 wt % of Cu, 0.005 to 0.06 wt % of Sol.Al, 0.015 wt % or less of P, and 0.020 wt % or less of S; additionally two or more elements selected from 0.002 to 0.025 wt % of Nb, 0.002 to 0.03 wt % of V, and 0.002 to 0.15 wt % of Co; and Fe and unavoidable impurities as the reminder,

wherein a microstructure of a central portion of the steel plate is formed as a mixed structure of 20 area % or more of tempered martensite and the residue bainite and wherein the steel plate has a thickness of 50 mm to 150 mm.

2. The pressure vessel steel plate of claim 1 , wherein the steel plate holds tensile strength of 550 MPa or more even when the PWHT is performed for up to 50 hours in the range of 600 to (Ac1-20) ° C.

3. The pressure vessel steel plate of claim 1 , wherein the steel plate has Charpy impact energy value of 100 J or more at −30° C. even when the PWHT is performed for up to 50 hours in the range of 600 to (Ac1-20)° C.

4. A manufacturing method of a pressure vessel steel plate having excellent resistant against a post weld heat treatment (PWHT), the manufacturing method comprising:

reheating and hot rolling a steel slab including 0.05 to 0.17 wt % of C, 0.59 to 1.00 wt % of Si, 0.3 to 0.8 wt % of Mn, 1.0 to 1.5 wt % of Cr, 0.3 to 1.0 wt % of Mo, 0.003 to 0.30 wt % of Ni, 0.003 to 0.30 wt % of Cu, 0.005 to 0.06 wt % of Sol.Al, 0.015 wt % or less of P, and 0.020 wt % or less of S; additionally two or more elements selected from 0.002 to 0.025 wt % of Nb, 0.002 to 0.03 wt % of V, and 0.002 to 0.15 wt % of Co; and Fe and unavoidable impurities as the reminder in the temperature range of 1000 to 1250° C.;

a primary heat treatment operation in which the hot rolled steel plate is held for a time of {1.3.times.t+(10 to 30)} (min) (wherein, t is thickness (mm) of steel) in the temperature range of 850 to 950° C.; and a tempering heat treatment operation in which the heat treated steel plate is maintained for a time of {1.6×t+(10 to 30)} (min) (wherein t is thickness (mm) of steel) at a temperature in a temperature range of 600° C. to the Ac1 point after the heat treated steel plate is cooled to martensite and bainite formation temperature ranges, and wherein the steel plate has a thickness of 50 mm to 150 mm.

5. The manufacturing method of claim 4 , further comprising, after the tempering heat treatment operation, a process of performing the PWHT for up to 50 hours in the range of 600 to (Ac1-20) ° C.

6. The manufacturing method of claim 4 , wherein after the tempering heat treatment operation, a microstructure of the steel plate is formed as a mixed structure of 20 area % or more of tempered martensite and the residue bainite.

7. The manufacturing method of claim 4 , wherein in the hot rolling of the steel slab, a reduction ratio per pass is 5 to 30%.

8. The manufacturing method of claim 4 , wherein in the cooling of the steel plate, a cooling speed is 2.5 to 30° C./sec.

9. The manufacturing method of claim 4 , wherein the steel plate after the tempering heat treatment operation holds tensile strength of 550 MPa or more even when the PWHT is performed for up to 50 hours in the range of 600 to (Ac1-20) ° C.

10. The manufacturing method of claim 4 , wherein the steel plate after the tempering heat treatment operation has Charpy impact energy value of 100 J or more at −30° C. even when the PWHT is performed for up to 50 hours in the range of 600 to (Ac1-20) ° C.